1. Field of the Invention
The present invention relates to the regeneration of catalysts employed in a fluid catalytic cracking process. More particularly, this invention relates to combusting CO to CO.sub.2 at the interface between the dense phase catalyst bed and the dilute catalyst phase by passing a minor portion of the regeneration gas through a distribution means located at said interface.
2. Description of the Prior Art
The fluidized catalytic cracking of hydrocarbons is well known in the prior art and may be accomplished in a variety of processes which employ fluidized solid techniques. Normally in such processes, suitably preheated, relatively high molecular weight hydrocarbon liquids and/or vapors are contacted with hot, finely divided, solid catalyst particles either in a fluidized bed reaction zone or in an elongated riser reaction zone, and maintained at an elevated temperature in a fluidized state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons typical of those present in motor gasolines and distillate fuels.
During the cracking reaction, coke is deposited on the catalyst particles in the reaction zone, thereby reducing the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stock. In order to restore a portion, preferably a major portion, of activity to the coke contaminated or spent catalyst, the catalyst is transferred from the reaction zone into a regeneration zone wherein the catalyst is contacted with an oxygen-containing regeneration gas, such as air, under conditions sufficient to burn at least a portion, preferably a substantial portion, of the coke from the catalyst. The regenerated catalyst is subsequently withdrawn from the regeneration zone and reintroduced into the reaction zone for reaction with additional hydrocarbon feed. Commonly, spent catalyst from the reaction zone is passed therefrom to a stripping zone for removal of strippable hydrocarbons from the catalyst particles prior to transferring the catalyst to the regeneration zone.
Typical regeneration zones in the prior art comprise large vertical cylindrical vessels wherein the spent catalyst is maintained as a fluidized bed by the upward passage of an oxygen-containing regeneration gas. The fluidized catalyst forms a dense phase catalyst bed in the lower portion of the vessel and a dilute catalyst phase containing entrained catalyst particles above, with an interface existing between the two phases. Flue gas, which normally comprises gases arising from the combustion of the coke on the spent catalyst, inert gases such as nitrogen from air, any unconverted oxygen and entrained catalyst particles, is then passed from the dilute catalyst phase into solid-gas separation means within the regeneration zone to prevent excessive losses of the entrained catalyst particles. The catalyst particles separated from the flue gas are returned to the dense phase catalyst bed. A substantially catalyst-free flue gas may then be passed from the separation means to equipment downstream thereof, for example to a plenum chamber, or be discharged directly from the top of the regeneration zone. Cyclone separators are commonly used as separation means.
The burning of coke deposits from the catalyst in the regeneration zone may be characterized in a simplified manner as the oxidation of carbon and represented by the following chemical equations: EQU C+O.sub.2 .fwdarw.CO.sub.2 ( 1) EQU 2C+O.sub.2 .fwdarw.2CO (2) EQU 2CO+O.sub.2 .fwdarw.2CO.sub.2 ( 3)
Reactions (1) and (2) both occur under typical catalyst regeneration conditions wherein the catalyst temperatures may range from about 1050.degree. to about 1300.degree. F. and are exemplary of gas-solid chemical interactions when regenerating catalyst at temperatures within this range. The effect of any increase in temperature is reflected in an increased rate of combustion of carbon and a more complete removal of coke from the catalyst particles. Gas phase reaction (3) is also accelerated by increased temperature as well as higher pressure and, particularly, the amount of oxygen present. Somewhat lower temperatures may be employed where an added carbon monoxide combustion catalyst or promoter is employed. The promoter may be incorporated into the catalyst or introduced into the regeneration zone separately. In addition to the above reactions which relate to the formation of carbon monoxide and carbon dioxide from carbon, water is formed from hydrogen in the coke.
One difficulty which arises in regenerating catalyst is that as the coke is burned with oxygen, it tends to produce substantial amounts of carbon monoxide which is subject to further oxidation into carbon dioxide as represented by reaction (3) above, an exothermic reaction. The use of an amount of oxygen theoretically sufficient to burn coke in the fluidized catalyst bed to a desired low level of residual coke upon regenerated catalyst has had the frequent undesirable effect of evolving a combustible mixture of oxygen and carbon monoxide in the dilute catalyst phase which may undergo the further combustion commonly referred to as "afterburning". (See "Oil and Gas Journal", Vol. 53 (No. 3), pp. 93-94, 1955 for further discussion). The "afterburning" causes a substantial increase in the temperature of the dilute catalyst phase which may exceed about 1500.degree. F. Such high temperatures in the dilute catalyst phase can cause deactivation of the catalyst, thereby requiring additional catalyst replacement to the process in order to maintain a desired catalytic activity in the hydrocarbon reaction zone. Additionally, these high temperatures may cause damage to mechanical components of the regeneration zone, particularly in that portion of the regeneration zone in contact with the substantially catalyst-free flue gas wherein the temperature may increase to 1800.degree. F. or greater. Such excessive temperatures in the substantially catalyst-free flue gas occur when reaction (3) does not proceed at a sufficient rate in the dense bed phase and in the dilute catalyst phase such that said flue gas contains an excess of oxygen and carbon monoxide so as to cause afterburning to be initiated. Reaction (3) then proceeds rapidly within the substantially catalyst-free flue gas since there is very little entrained catalyst present to absorb the heat released, and thereby reduce the rise in temperature. Thus, in that portion of the regeneration zone wherein the flue gas is substantially catalyst-free, there will occur a rapidly accelerating rise in temperature due to the heat released as complete combustion of carbon monoxide occurs or as the available oxygen is utilized, in the absence of any means to moderate the temperature therein.
Several methods have been proposed to overcome undesirable afterburning in that portion of the regeneration zone containing the substantially catalyst-free flue gas. One method used in industry is to control the oxygen-containing gas stream entering the regeneration zone directly responsive to a predetermined temperature differential between the outlet of the dilute catalyst phase and the dense bed of the regeneration zone in order to minimize excess oxygen therein. This practice eliminates excessive temperatures in that portion of the regeneration zone in contact with the substantially catalyst-free flue gas and produces a small amount of oxygen in said flue gas, generally less than about 0.5 vol. %. However, this procedure has limited effect in attaining low levels of residual carbon on regenerated catalyst at conventional fluid catalytic cracker operating conditions wherein the dense phase bed temperature ranges from about 1050.degree. to about 1250.degree. F. and the amount of uncombusted carbon monoxide ranges from about 6-12 vol. %. (See, for example, U.S. Pat. No. 3,206,393.)
When catalyst regeneration is carried out to obtain a regenerated catalyst with low residual carbon content and a low carbon monoxide level in the flue gas, there have been, in general, three approaches suggested to overcome excessively high temperatures in the dilute catalyst phase. In one approach, a cooling medium which may comprise steam, liquid water, unregenerated catalyst, hydrocarbon oil, flue gas, etc. is injected to cool the dilute catalyst phase below a temperature which will cause damage to the catalyst or to mechanical members of the regeneration zone (see, for example, U.S. Pat. Nos. 2,393,839, 2,454,373 and 2,580,827). Another approach is to employ series catalyst regeneration wherein the catalyst to be regenerated is contacted in a plurality of dense phase regeneration zones with an oxygen-containing regeneration gas in which the catalyst flows from zone to zone, the temperature in each zone not exceeding a temperature at which excessive afterburning will occur. (See, for example, U.S. Pat. Nos. 2,788,311, 3,494,858, and 3,563,911.) The third approach involves the use of indirect heat exchange, such as steam generation coils in the dense phase bed. When such methods are employed in conventional fluid catalytic cracker operations, the amount of carbon monoxide present in the flue gas may still be substantial, generally being in the range of from 6-12 vol. %. In addition, the above methods result in the loss of recoverable heat from the process or require expenditures for the use of additional equipment.
More recently, it has been suggested to reduce the levels of both residual carbon on regenerated catalyst and emissions of carbon monoxide by operating the dense phase bed in the regeneration zone at elevated temperatures, that is, temperatures ranging from about 1250.degree. to about 1400.degree. F. These high dense phase bed temperatures can result in substantial afterburning in the dilute catalyst phase and may be prevented by techniques similar to and having the same disadvantages as those mentioned above. As an example, U.S. Pat. No. 3,909,392 discloses a two-stage process for regenerating fluid catalytic cracking catalysts at elevated temperatures, thereby favoring substantially complete combustion of carbon monoxide, with a provision for recovering the heat evolved in the dilute catalyst phase by the use of a circulating stream of partially regenerated catalyst.
As another example, U.S. Pat. Nos. 3,844,973, 3,919,115 and 3,923,686 disclose a regeneration zone which comprises a first dense bed, a dilute phase transport riser, and a second dense bed. The carbonaceous deposits are oxidized in the first dense bed to produce a partially spent regeneration gas containing carbon monoxide and a regenerated catalyst. The regeneration gas and regenerated catalyst are then passed to a dilute phase transport riser wherein, preferably, carbon monoxide is combusted to carbon dioxide, with the regenerated catalyst being passed to the second dense bed from which it is returned to the reaction zone. Thus, the oxidation of both carbon monoxide and coke occur within the same regeneration zone but, preferably, at different locations. These references disclose control of the temperature of the regenerated catalyst returning to the reaction zone independently of the coke oxidation and as another variation, control of the coke on spent catalyst to a predetermined residual level by adjusting the regeneration gas rate to the first dense bed.
Also, some published foreign patent applications making claim to priority applications filed in the U.S. suggest controlling the oxidation of carbon monoxide to carbon dioxide in the dense bed phase by regulating the oxygen rates passing into the regeneration zone in order to reduce the level of coke on regenerated catalyst leaving the regeneration zone and to minimize, if not totally eliminate, afterburning occuring within the regeneration zone. (See, for example, German application No. 2,327,209 and Netherlands application No. 73,09759.) However, for a commercially operating fluid catalytic cracking unit, any appreciable increase in regeneration gas rate might require additional blower or compressor capacity, thereby necessitating an added expenditure. In addition, although the low residual carbon on regenerated catalyst and the low carbon monoxide level in the flue gas can be maintained by adjusting the regeneration gas rate and hence the oxygen concentration in the regeneration zone, this method has the disadvantage of involving the interaction of several process variables. For example, changes in the regeneration gas rate, while maintaining the excess oxygen in the regeneration zone at a constant level, will result in a directly proportional heat release in said zone. The differential heat evolved will cause a change in the dense phase bed temperature which in turn must be compensated for by adjusting the catalyst circulation rate. This changes the temperature in the reaction zone which in turn causes a variation in the coke make which requires complicated secondary corrective measures.
Thus, in view of the disadvantages of the methods employed in the foregoing prior art, it would appear desirable to have available a simple and convenient technique for reducing the carbon monoxide content in the flue gas to desirable low levels to minimize or eliminate excessive afterburning, particularly in that portion of the regeneration zone occupied by substantially catalyst-free flue gas.